A power semiconductor device is a semiconductor used as a switch or rectifier in power electronics. With the development of metal oxide semiconductor field-effect transistor (MOSFET) and insulated-gate bipolar transistor (IGBT), power semiconductors are more widely used in power devices, e.g., switch mode power suppliers, power amplifiers, and power converters. For a power device, the power dissipated by a power semiconductor is dissipated as heat, which causes the temperature of the power semiconductor to rise. When a junction temperature of the power semiconductor reaches a predetermined maximum junction temperature TJMAX, the power device overheats and fails. The junction temperature is the barrier layer temperature of the transistors. In some circumstances, power dissipation is unevenly distributed among power semiconductors of power devices. In order to prevent the failure, the maximum power capacity of the power device must be overly conservative to account for the uneven distribution of power dissipation among the power semiconductors.
One example of such power device is a gradient power amplifier in magnetic resonance imaging (MRI) systems, which generate magnetic field in X, Y, and Z three-axis in order to provide the necessary spatial resolution for the reception of the magnetic resonant signals emitted by the hydrogen protons of an examination object. Various topologies for the gradient amplifier have been proposed to deliver a specified current profile to the gradient coil, such as cascaded full bridge topology with multiple full bridge circuits in a stack configuration, or multiple bridge power conversion topology with multiple full bridge circuits connected in parallel at their input/outputs.
FIG. 1A illustrates a schematic diagram of a conventional full bridge circuit 100. FIG. 1B illustrates an exemplary current profile 110 flowing through the gradient coil. The modulation scheme of the full bridge circuit 100 used in gradient amplifiers of various topologies controls the current profile 110 flowing through the gradient coil. For the purpose of brevity, only full bridge circuit 100 is shown in FIG. 1A to illustrate the relationship between the modulation scheme of full bridge circuits and the current profile. However, those skilled in the art would understand that the gradient amplifiers may include multiple full bridge circuits as aforementioned.
Referring to FIG. 1A, the full bridge circuit 100 includes high side power semiconductors 101 and 103, and low side power semiconductors 105 and 107, e.g., IGBTs. The high side and low side power semiconductors 101 and 105 are connected in series at node A to form a first bridge leg. Similarly, the high side and low side power semiconductors 103 and 107 are connected in series at node B to form a second bridge leg. A gradient coil 109 is connected between nodes A and B. The current profile 110 flowing through the gradient coil 109 is illustrated in FIG. 1B. As shown in FIG. 1B, the current profile 110 represents a periodic waveform. During a time duration of T1 of a cycle time, the modulation scheme switches on the high side power semiconductor 101 and the low side power semiconductor 107 and switches off the high side power semiconductor 103 and the low side power semiconductor 105. Due to the modulation scheme of the full bridge circuit 100, the current profile 110 flows through the high side power semiconductor 101, the gradient coil 109 and the low side power semiconductor 107 with a current level I1 during the time duration of T1. During a time duration of T2 of the cycle time, the modulation scheme switches off the high side power semiconductor 101 and the low side power semiconductor 107 and switches on the high side power semiconductor 103 and the low side power semiconductor 105. Due to the modulation scheme of the full bridge circuit 100, the current profile 110 flows through the high side power semiconductor 103, the gradient coil 109 and the low side power semiconductor 105 with a current level I2. As such, power dissipation across each of the power semiconductors 101 through 107 is determined by the current level and time duration of the current flowing through each power semiconductor. More specifically, the average power dissipation across each of the power semiconductors 101 and 107 is given by equation (1):Pavg1=(Kcond×I1+Ksw×I1)×D  (1)The average power dissipation across each of the power semiconductors 103 and 105 is given by equation 2:Pavg2=(Kcond×I2+Ksw×I2)×(1−D)  (2)where Kcond is the conduction loss coefficient of the power semiconductors, Ksw is the switching loss coefficient of the power semiconductors and D is the duty cycle of the modulation scheme given by equation (3)D=T1/(T1+T2)  (3)According to equations (1) and (2), the power dissipation of the full bridge circuit 100 is unevenly distributed among the power semiconductors 101 through 107 for the current profile 110 with long time duration T1 and short time duration T2. Uneven power dissipation can lead to different temperature rise across each power semiconductor according to equation (4):Z(t)=(TJ(t)TC(t))/P  (4)where Z(t) represents the thermal impedance at time t, TJ(t) represents the junction temperature at time t, TC(t) represents the case temperature of the module case in operation at time t, and P represents the power dissipation. Since each semiconductor is packaged inside the gradient amplifier, they share a common case temperature Tc(t). Due to uneven power dissipation with long time duration T1 and short time duration T2 of the current profile 110, the temperature of the power semiconductors 101 and 107 will reach the predetermined maximum junction temperature TJMAX much earlier than those of the power semiconductors 103 and 105. To prevent failure of power semiconductors, gradient amplifier's maximum output capacity is limited to account for different temperature rise resulting from uneven power dissipation.